1. Field of the Invention
The present invention relates to a noise removing apparatus. More specifically, the present invention relates to an apparatus for removing a noise suited for removing a pulsive noise included in an FM stereo signal.
2. Description of the Prior Art
It has been well known that a pulsive noise such as an ignition noise generated by an automobile could interfere with normal reception by an FM receiver. Since such pulsive noise serves to phase modulate the FM signal, the same cannot be removed even by the use of a limiter and hence is transferred to a subsequent stage in the receiver after detection by a detector. Accordingly, it is necessary to remove such pulsive noise in a signal transmission path subsequent to a detector.
Referring to FIG. 1, there is shown a block diagram of an FM radio receiver employing a typical noise removing apparatus where the present invention can be advantageously employed. Referring to FIG. 1, the FM radio receiver shown comprises an antenna 101 for receiving a broadcast FM signal wave, a radio frequency amplifier 103 for amplifying the FM signal received by the antenna 101, a local oscillator 107 for generating a local oscillation signal for the purpose of frequency conversion, a frequency converter 105 for mixing the amplified FM signal from the radio frequency amplifier 103 with the local oscillation signal for converting the frequency of the FM signal into an intermediate frequency, an intermediate frequency amplifier 109 for amplifying the intermediate frequency signal from the frequency converter 105, an FM detector 111 for demodulating the intermediate frequency signal into the original low frequency signal, a stereo demodulating circuit 113 for demodulating the low frequency signal from the FM detector 16 into the original stereo signal, left and right audio frequency amplifiers 115 and 117 for amplifying the demodulated stereo left and right signals, and left and right loud speakers 119 and 121 for converting the ampified left and right audio frequency signals into the left and right sounds. Detailed structure and operation of the respecitive blocks are well known to those skilled in the art. Hence, it is not believed necessary to describe the same here in more detail.
In the FM stereo receiver shown, the output of the detector 111 is applied through a noise removing circuit 2 to the stereo demodulating circuit 113. The present invention is directed to an improvement in such a noise removing apparatus as shown in FIG. 1 which is interposed between the FM detector 111 and the stereo demodulating circuit 113.
An example of a noise removing apparatus of interest is disclosed in, for example, Japanese Patent Publication No. 15710/1964, published for opposition Aug. 5, 1964 and entitled "SYSTEM FOR REMOVING PULSIVE NOISE IN SIGNAL". The system disclosed in the above referenced Japanese Patent Publication No. 15710/1964 comprises a gate circuit in a signal transmission path. Upon detection of a pulsive noise, the gate circuit is placed in an interrupted state for a predetermined time period, so that the pulsive noise may not pass through the signal transmission path to a subsequent stage. A capacitor is also coupled to the output terminal of the gate circuit to hold a level. The capacitor serves to hold or store the level of the signal immediately before the gate circuit is interrupted. Since the above described prior art serves to supplement the signal voltage level stored in the capacitor when the gate circuit is interrupted, an advantage is brought about that a noise component is removed while the signal is less disturbed during the gate time period. Therefore, the above described prior art can be advantageously employed in reception of FM monaural broadcasting, in particular.
However, in the case where the above described prior art is utilized in reception of the FM stereo broadcasting, a new problem occurs that a stereo pilot signal is lost. Several types of noise removing apparatuses adapted for preventing a stereo pilot signal from being lost and hence suited for reception of the FM stereo broadcasting, in particular, have been proposed and put into practical use. One type of such apparatus of interest is disclosed in the U.S. Pat. No. 3,739,285 issued June 12, 1973. Briefly described, the apparatus disclosed in the above referenced U.S. Pat. No. 3,739,285 comprises a capacitor which is the same as that of the previously described prior art and a parallel resonance circuit connected in series with the capacitor. The parallel resonance frequency of the parallel resonance circuit is selected to be the frequency of a pilot signal contained in an FM stereo broadcasting signal, say 19 kHz. Accordingly, the signal level immediately before the gate circuit interrupts the signal is held or stored and at the same time a pilot signal obtained from the parallel resonance circuit is applied to a stereo demodulating circuit. Since the apparatus is so structured, the gate circuit is opened upon detection of a pulsive noise, so that such pulsive noise component is prevented from being applied to the stereo demodulating circuit. Furthermore, since the signal level has been held in the capacitor when the gate circuit is thereafter closed again, continuity of the signal is established. Accordingly, the apparatus disclosed in the U.S. Pat. No. 3,739,285 can effectively remove a pulsive noise, while preventing the pilot signal for the stereo demodulating circuit from being lost. Although the above referenced U.S. Pat. No. 3,739,285 has been improved in that the stereo pilot signal is prevented from being lost, another resonance is caused by the holding capacitor and the parallel resonance circuit. More specifically, one series resonance circuit is formed by a capacitor for holding the signal level and a parallel resonance circuit and the series resonance frequency thereof causes a distortion in a signal being applied to the stereo demodulating circuit. The frequency causing such distortion is necessarily lower than the resonance frequency of the parallel resonance circuit and falls in an audible frequency region, with the result that a distorted sound is naturally heard from a speaker. Such series resonance causes further problems.
In order to eliminate the above described problems, the same assignee as that of the present invention previously proposed a novel noise removing apparatus which was improved over the above referenced U.S. Pat. No. 3,739,285 and issued Jan. 3, 1978 as U.S. Pat. No. 4,066,845. The above referenced U.S. Pat. No. 4,066,845 is adapted to prevent a stereo pilot signal from being lost by means of an oscillation circuit during a time period when a gate circuit is interrupted.
Another circuit for removing a pulsive noise of interest is proposed in Japanese Utility Model Laying Open No. 106608/1978 laid open Aug. 26, 1978 for public inspection. The pulsive noise removing apparatus disclosed in the above referenced Japanese Utility Model Laying Open No. 106608/1978 comprises a trap circuit. More specifically, in such a noise removing apparatus as shown in the above referenced Japanese Patent Publication No. 15710/1964, a stereo composite signal is provided wherein a stereo pilot signal has been removed by a trap circuit. More specifically, a stereo pilot signal is removed in advance from a stereo composite signal by means of a trap circuit, whereupon the stereo composite signal with the stereo pilot signal removed is applied to the gate circuit. The above described noise removing apparatus has eliminated such an adverse influence that a stereo pilot signal is lost because of a signal level holding capacitor, inasmuch as no stereo pilot signal exists in a signal transmission path. However, insertion of a trap circuit in a signal transmission path increases the width or duration of a pulsive noise, which necessitates an increased interruption period by a gate circuit. Accordingly, a disadvantage is encountered that discontinuity of a stereo composite signal is undesirably increased.